The present invention relates to an output distributing/synthesizing apparatus in a current/spare operation scheme or parallel operation scheme using TV (Television) and FM (Frequency Modulation) broadcasting units and the like.
FIGS. 4A and 4B show the schematic arrangements of conventional output switching apparatuses. The output switching apparatus shown in FIG. 4A is disclosed in Japanese Patent Laid-Open No. 57-197901. This apparatus is constituted by 4-terminal hybrid circuits 21 and 22 and variable phase shifters 41 and 42 connected between the output terminals of the 4-terminal hybrid circuit 21 and the input terminals of the 4-terminal hybrid circuit 22. The output switching apparatus having this arrangement switches outputs from first and second broadcasting units 26 and 28 to an antenna 27 and a pseudo-antenna 29 without interruption. Reference numeral 43 denotes a quarter-wavelength line for a phase shift of 90xc2x0.
FIG. 4B shows an output switching circuit for switching output signals from first and second broadcasting units 26 and 28 to antennas 27a and 27b without interruption. This circuit differs from that shown in FIG. 4A in that 4-terminal switches 71 to 74 are arranged in twos between the first and second broadcasting units 26 and 28 and a 4-terminal hybrid circuit 21 and between a 4-terminal hybrid circuit 22 and the antennas 27a and 27b, and the pseudo-antenna is replaced with the antenna.
The first problem posed by the output switching apparatus having the arrangement is that while a program is put on the air in the first broadcasting unit 26, the second broadcasting unit 28 cannot perform maintenance using a test signal and the pseudo-antenna 29.
The reason for this is that since the degree of isolation between the first and second broadcasting units 26 and 28 is only about xe2x88x9240 dB at the antenna output terminal, the test signal from the second broadcasting unit 28 may be superimposed on the signal from the first broadcasting unit 26 while the program is on the air.
The second problem is that in the 2-line feeding system using two antennas, the plurality of 4-terminal switches 71 to 74 are required to independently switch output signals from the first and second broadcasting units 26 and 28 to the antennas 27a and 27b, as shown in FIG. 4B.
It is an object of the present invention to provide a distributing/synthesizing apparatus which can attain a reduction in size of a broadcasting system for current/spare operation and parallel operation, a simplification of the circuit arrangement, and an improvement in maintainability.
In order to achieve the above object, according to the present invention, there is provided a signal distributing/synthesizing apparatus comprising signal distributing/synthesizing means for distributing a signal input to a first terminal to second and third terminals, and synthesizing signals input to the second and third terminals into a signal to be output to the first terminal, first and second antennas forming a radio transmission channel for an output signal from the signal distributing/synthesizing means, a pseudo-antenna forming a pseudo-radio transmission channel for an output signal from the signal distributing/synthesizing means, and first, second, and third switching means, wherein the first switching means complementarily switches/connects a first broadcasting unit and the second antenna to the first terminal of the signal distributing/synthesizing means and the third switching means, the second switching means complementarily switches/connects a second broadcasting unit and the second antenna to the second terminal of the signal distributing/synthesizing means and the third switching means, and the third switching means complementarily switches/connects the pseudo-antenna and the third terminal of the signal distributing/synthesizing means to the first and second switching means.